1. Field of the Invention
The present invention relates to a sludge treating apparatus. More specifically, the present invention relates to a sludge treating apparatus for treating a raw sludge produced by waste water treatment through an anaerobic digestion process.
2. Description of the Prior Art
Conventionally an approach has been taken in which a raw sludge produced by a waste water treatment is digested under an anaerobic condition for the purpose of conversion thereof to a digested sludge which is suited for post-treatment. A conventional typical example is that a raw sludge obtained from a settling tank included in a waste water treating apparatus is introduced into an anaerobic digestion tank provided on the ground and the same is retained in the anaerobic digestion tank for a predetermined period of time or a predetermined number of days, say thirty days for the purpose of digestion. The raw sludge produced from the waste water treating apparatus is usually subjected to gravity condensation so that the raw sludge may be condensed to the sludge concentration of approximately 2 to 3%, whereupon the raw sludge is introduced into an anaerobic digestion tank provided on the ground. Such a conventional sludge treating system as described above involved a problem to be solved as set forth in the following.
In the case of the above described conventional system, the condensation ratio of the raw sludge before the same is introduced into an anaerobic digestion tank is small and accordingly a gravity condensation tank and an anaerobic digestion tank each of a large capacity were required. More specifically, the condensation ratio being achieved by a conventional gravity condensation is 2 to 3% at the most as described previously. Accordingly, the ratio of volume reduction due to the gravity condensation is merely 1/2 to 1/3 at the most. The average retention time in the gravity condensation tank is approximately twelve hours and accordingly the capacity for twelve-hour retaining is required as a capacity of the gravity condensation tank. On the other hand, an average number of days required for digestion in the anaerobic digestion tank is considered to be approximately thirty days. Therefore, the anaerobic digestion tank needs a capacity for retaining the raw sludge as gravity condensed for thirty days. In addition, a predetermined temperature say 35.degree. C. is required for methane fermentation in the anaerobic digestion tank. Therefore, usually the raw sludge is in advance heated before the same is introduced into the anaerobic digestion tank and the warmed raw sludge is introduced into the anaerobic digestion tank. Since a conventional system for introducing a raw sludge as gravity condensed into an anaerobic digestion tank has a small ratio of condensation, energy of large quantity was required for warming the raw sludge in advance. For example, assuming that the ratio of condensation in the gravity condensation tank is 2% and the ratio of capacity reduction is 1/2, it requires that a half of the raw sludge produced day by day from a waste water treating apparatus is warmed to a predetermined temperature. Thus, it would be readily appreciated how a large amount of energy is required for that purpose. In addition, a conventional anaerobic digestion tank is usually installed on the ground. Accordingly, thermal dissipation from the anaerobic digestion tank is large and surplus energy is required in order to maintain the sludge in the tank to a temperature required for methane fermentation. More specifically, according to a conventional anaerobic digestion tank, even if reaction heat is generated as a function of methane fermentation in the tank, dissipation of heat is larger, as described above, and accordingly the temperature in the tank is lowered after all. In order to maintain the temperature in the tank constant, therefore, more energy is required.
Furthermore, according to a conventional system, the concentration of the raw sludge as condensed by the gravity condensation tank is primarily dependent on the nature of the raw sludge and therefore it is difficult to obtain a uniformly condensed sludge. Accordingly, it follows that the degree of reaction in the digestion tank fluctuates and it could happen that the raw sludge is transferred to a post-treating apparatus before the same is fully digested. Under the circumstances, the benefit of digesting the sludge in advance so as to be suited for post-treatment is lost and reduced.
Meanwhile, conventionally a so-called pressurized floating condensation is also well known, apart from the above described gravity condensation. However, even in the case of a pressurized floating condensation, the ratio of condensation of the raw sludge is 4 to 5% at the most and accordingly the ratio of capacity reduction is also 1/4 to 1/5. In addition, the pressurized floating condensation ordinarily requires a dosage of a polymer coagulating agent, a tank, and a compressor with capacity more than 3-5 kg/cm.sup.2 discharge pressure for air dissolution in water. Accordingly this condensation method involves a disadvantage that the running cost is extremely expensive. In addition, even in the case of the pressurized floating condensation, the degree of condensation is primarily dependent upon the nature of the waste water and it is difficult or impossible to obtain a raw sludge of uniform concentration.